Economisers

by
Economisers
  1. An economizer is an heat exchanger which is used to raise the temperature of feed water with the help of waste heat energy of gases before it is supplied to the boiler.
  2. Note that the temperature of gases is about 350°C 550°C at exit of last heater/ superheater having large amount of heat energy which otherwise would have been wasted.
  3. Advantage of preheating the feed water is that it reduces the amount of heat to be added to the steam in boiler per kg of steam.
  4. Alternately, we may say that the rate of steam evaporation rises for the same rate of heat supplied by burning the fuel in the boiler.
  5. An economizer is shown in Figure which is similar to a convective type of superheater.
  6. It is generally placed between the CSH and air preheate.
An Economisers

Advantages of an economizer are :

1. Improves boiler efficiency. Approximately there is 1% rise in efficiency per 5°C rise in feed water temperature.
2. It reduces the temperature stresses due to reduced temperature differential in the boiler. Economiser is made of cast iron in case of low and medium pressure boilers or steel tubes of continuous loop type in case of high pressure boilers. To avoid water hammering due to boiling of water in the economizer made of cast iron, it is essential that feed water temperature is kept about 40°C below the saturation temperature of steam. The energy equation for economizer can be written as :

Q = mW . CPW . (tWO – tWI)
= mg . Cpg x (tg1 – tg2) = UO
. (△Tlm)

Where, △T is the log mean temperature difference. The heating surface area, Ao, of economizer can be increased by providing fins on the gas side since heat transfer coefficient on gas side is less than the heat transfer coefficient on water side.

Design aspects of an economizer are :
1. The gas should come in contact with the entire heating surface area.
2. Heating surface must be polished to reduce accumulation of dust.
3. Any steam formation with water in tubes should escape into boiler drum without causing any water hammer.
4. The gas passages are so designed so as to maintain constant gas velocity throughout the passages. These passages should be straight to avoid formation of eddies and pockets for collection of dust.
5. It should be compact and easy to carry out cleaning and maintenance.
6. Water velocity should not be below 0.5 m/s in non-steaming economizers and below 1 m/s in steaming economizers.

Leave a Comment